Many cases arise in which there is a need to move an object at a controlled rate. One example is the case of printing high resolution images with a thermal printer. In some applications of thermal printers, a receiver (i.e., printing media) of an image is moved either continuously or in a series of steps as an image is formed on the receiver. In one type of thermal printer a modulated laser beam is focused and scanned across the receiver and an overlying patch of dye-donor film. The receiver is moved an incremental distance between each scan of the laser as an image is produced on a line-by-line basis. A thermal printer of this type is disclosed in a U.S. patent application Ser. No. 457,593 (S. Sarraf, filed Dec. 27, 1990) described in the Cross Reference to Related Patent Applications section hereinabove.
In another type of printer, a receiver is moved continuously while a modulated laser beam is scanned back and forth in a direction that is substantially, but not quite, orthogonal to the direction of movement of the receiver.
In both of these types of printers the receiver is placed on a stage which is driven by a rotating lead screw. In order to produce a high resolution image (i.e., 2000 dots per inch or higher) without visible distortions, it has heretofore been necessary to use a very high precision lead screw in these applications. High precision lead screws are produced by a process of grinding and lapping. These processes produce a lead screw which has a very low cumulative error and a very low cyclic error. A lead screw with a low cumulative error has a highly predictable relationship between displacement along its length and number of revolutions of the screw. A cyclic error is one which appears repetitively in each revolution of the lead screw. A lead screw with a low cyclic error has a highly predictable relationship between displacement along its length and any portion of any one revolution of the screw.
High precision lead screws made by grinding and lapping are much more expensive than conventional lead screws which are produced in a thread rolling or cutting operation on a lathe. In the prior art, inexpensive conventional lead screws have been adapted for use in applications that require low cumulative error. This adaptation has been achieved by a process of developing a characterization of the lead screw and using the characterization to control a drive system in which the lead screw is employed to achieve a predictable relationship between displacement and number of revolutions of the conventional lead screw. This characterization process is performed by a manufacturer of the lead screw prior to sale of the lead screw. The lead screw is placed into a measurement apparatus which uses a laser interferometer to measure and record a relationship of displacement of a target (which is driven by the screw) and a number of revolutions of the screw. These measurements are made at a plurality of locations along the screw and the results are recorded in a memory device such as an EPROM (Electronically Programmable Read Only Memory). The EPROM is then shipped by the lead screw manufacturer, along with the lead screw. A user of the lead screw puts the EPROM into a controller of a drive system into which the lead screw is assembled. The controller drives the lead screw so that specific compensations are made for the cumulative errors of that particular screw.
However, adapting a conventional lead screw to operate without cumulative error does not make the lead screw useful for high resolution printing applications. In a high resolution printing application, it is necessary to use a lead screw which has a low cyclic error rate. The above described characterization process does not reduce cyclic errors in a conventional lead screw. Thus, high resolution printers have heretofore continued to use expensive high precision lead screws.
The high cost of a precision lead screw is a relatively unimportant factor in a printer which is designed for a high volume and high production application. Such large volume printers typically cost hundreds of thousands of dollars. In this context, a precision lead screw which costs between $500 and $1000 dollars is a readily affordable component.
However, there are many types of printers which are intended for use in desk top situations. These printers are typically designed to be used in conjunction with a personal computer to produce images generated by various types of graphics software in the computer. These desk top printers are usually sold for prices less than about $5000. In this context, a high precision lead screw that costs $500 to $1000, is a component with a prohibitively high cost.
Consequently, desk top printers have heretofore been limited to relatively low resolution application (300 dots per inch or less).
It is desirable to have a thermal printer with a low cost lead screw that is capable of producing high resolution images.